From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-90186][LOW] null_blk: reject per-device queue resize for shared tag set [ Upstream Date: Thu, 17 Sep 2026 14:51:39 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-90186 X-AL-KERNEL-Priority: LOW X-AL-KERNEL-Severity: LOW X-AL-KERNEL-Base-Severity: LOW X-AL-KERNEL-KPANIC: NO X-AL-KERNEL-ActionableScore: 0 X-AL-KERNEL-ActionableScore-Lower: unknown X-AL-KERNEL-Commit: 9c7bae099ddc92676ad1b7e29dd6c089e5f6196c List-Id: CVE: CVE-2026-90186 Priority: LOW AL-KERNEL base severity: LOW KPANIC flag: NO Patch: null_blk: reject per-device queue resize for shared tag set [ Upstream Commit: 9c7bae099ddc92676ad1b7e29dd6c089e5f6196c Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9c7bae099ddc92676ad1b7e29dd6c089e5f6196c Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90186 Analysis date: Thu, 17 Sep 2026 14:51:39 -0400 ActionableScore: 0 ActionableScore lower bound: unknown Actionable bucket: Ordinary Moderate Manual review required: NO Summary: A host administrator can crash the kernel by resizing per-device null_blk queues while shared_tags uses a global tag set, causing mq_map entries to point at NULL hctx slots and triggering a NULL pointer dereference in blk_mq_map_swqueue. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-90186 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90186 The original announcement does not normally provide a security severity estimate, CVSS assessment, or enough information to determine whether the reported kernel bug represents a practically relevant security issue. This report was generated by AL-KERNEL, an AI-assisted Linux kernel vulnerability analysis system developed by Alexander Larkin. It combines an autonomous classifier with LLM-assisted technical analysis and a separate ActionableScore mechanism. The purpose of this report is to prioritize Linux kernel CVEs before manual review, identify cases that require prompt investigation, and support automatic closure of issues that are unlikely to have meaningful security impact. Published priority for this report: LOW Manual review required: NO A detailed explanation of the methodology and priority rules is included at the end of this message. ====================================================================== AL-KERNEL CLASSIFICATION RESULT ====================================================================== CVE-2026-90186 LOW https://www.kernelcve.org/list/?q=CVE-2026-90186 CHECK WITH IMPACT FROM ORIG NN LOW Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';**CWE-476;*CWE-20;CWE-754;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'A NULL pointer dereference in null_blk can occur when a device uses shared_tags and a per device submit_queues or poll_queues resize is requested through configfs. The shared tag set is mapped using module wide queue counts, while the resize path can shrink set nr_hw_queues and leave mq_map entries pointing at NULL hctx slots. A later blk_mq_map_swqueue dereference of hctx cpumask can crash the kernel and cause denial of service. For the CVSS the PR:L value is used only for the paranoid score, because some test or delegated administration environments may allow a non root service account or container admin to control null_blk configfs knobs. In typical production systems PR:H is more realistic because loading null_blk and writing these configfs attributes require administrator privileges. The issue is not network reachable and there is no evidence of memory corruption beyond NULL dereference.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 0) YES KASAN NULLPTR HARDWARE DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3 NO NO checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=0 PrimitiveClass=CLASS_C RealUAF=NO HostAdminRequired=YES NamespaceScopedPrivilege=NO ImportantCandidate=NO ManualReview=NO AutoDowngradeAllowed=YES ## 1. ActionableScore * Raw conservative score: -1 * Conservative score: 0 * Raw paranoid score: -1 * Paranoid score: 0 * Applied floor/cap: raw score clamped to 0 because ActionableScore is non-negative. * Final recommended bucket: **Ordinary Moderate**:: ## 2. Signal breakdown * +1 Reliable kernel crash / strong DoS: the supplied trace shows a NULL pointer dereference in `blk_mq_map_swqueue()` through `hctx->cpumask`, causing a kernel crash. * -2 Host-admin-equivalent privilege required: the supplied reproducer requires `modprobe null_blk`, configfs device creation, powering the device, and writing queue attributes under `/sys/kernel/config/nullb/...`, all of which are host-admin-style management operations in normal semantics. * +0 Weak corruption concern only: this is an invalid NULL pointer dereference caused by inconsistent queue mapping state, not a demonstrated useful memory corruption primitive. Important non-awarded signals: * RealUAF not awarded: no freed object or stale post-lifetime object use is established. * Concrete repeated release/refcount not awarded: there is no first release that may end lifetime followed by a second lifetime mutation on the same dangling reference. * Controlled reclaim not demonstrated. * Arbitrary or directed kernel write not demonstrated. * Confidentiality and integrity not awarded: the evidence supports crash only. * Extra availability not awarded: the same kernel crash is already counted once. * Remote reachability not established. * NamespaceScopedPrivilege not awarded: configfs/null_blk management is not shown to be fully attacker-reachable inside user namespaces. ## 3. Reachability analysis The demonstrated trigger is local host control-plane configuration of the `null_blk` test block driver. The reproducer loads the module with `shared_tags=1`, creates a configfs device, powers it on, and then writes a per-device queue count. This is host-administrator behavior. An ordinary unprivileged local user is not shown to be able to perform the complete trigger chain. A delegated service account or container administrator could matter in a specific product, but the supplied evidence does not establish such delegation. The complete operation is not shown to be namespace-scoped. No network path is involved. No special physical hardware is required because `null_blk` is a software block driver, but it is still an admin-controlled test/configuration subsystem. There is no race requirement. The crash follows from an ordered invalid configuration sequence where per-device queue resize is allowed despite use of a global shared tag set. ## 4. Primitive and severity interpretation The exact primitive is an invalid NULL pointer dereference. With `shared_tags` enabled, the device uses the global tag set. A per-device resize can shrink `set->nr_hw_queues`, leaving some `queue_hw_ctx` slots NULL while `mq_map[]` can still map CPUs to those slots. `blk_mq_map_swqueue()` later dereferences the NULL `hctx`. This is not an active structured UAF and not fixed stale storage after free. It is a consistency bug in block multiqueue mapping state that produces a NULL dereference. The demonstrated consequence is host kernel crash / denial of service. A speculative stronger consequence is not supported by the patch or trace. PrimitiveClass is CLASS_C because the behavior is crash/DoS from invalid state without an independently useful corruption, ownership, authorization, or lifetime primitive. ImportantCandidate is NO because there is no UAF, no repeated lifetime mutation, no controlled corruption, no security-boundary bypass, and the trigger is host-admin configuration. AutoDowngradeAllowed is YES because automatic downgrade would not discard an unresolved Important-grade primitive. Likely operational severity is ordinary Moderate or Low-like in kernel-security triage, despite a high availability impact in a strict CVSS-style model. ## 5. One-sentence report phrase A host administrator can crash the kernel by resizing per-device null_blk queues while shared_tags uses a global tag set, causing mq_map entries to point at NULL hctx slots and triggering a NULL pointer dereference in blk_mq_map_swqueue. ## 6. Manual review recommendation NO MANUAL CHECK NEEDED Reason: the issue is a host-admin-triggered NULL dereference in a configuration path, with no UAF, no repeated refcount/lifetime mutation, no controlled corruption, no remote reachability, and no authorization or isolation bypass. ## 7. Call-site confidence Call-site confidence: high ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: null_blk: reject per-device queue resize for shared tag set [ Upstream Commit: 9c7bae099ddc92676ad1b7e29dd6c089e5f6196c Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9c7bae099ddc92676ad1b7e29dd6c089e5f6196c Changed files: drivers/block/null_blk/main.c Diff excerpt: Not included in this email. See the upstream URL for the full patch. Full patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9c7bae099ddc92676ad1b7e29dd6c089e5f6196c ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-90186 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90186 The original CVE announcement normally does not include a security-level estimate. In particular, it may not contain a CVSS assessment, an impact level, or enough information to determine whether the reported bug is a practically relevant security issue. One purpose of this parallel CVE list is to provide that missing technical and prioritization information. The original goal of the AL-KERNEL project was to prioritize Linux kernel CVE analysis automatically before manual review. The system can also help identify non-security issues that may be suitable for automatic closure. This report was generated by AL-KERNEL, an AI-assisted Linux kernel vulnerability analysis system developed by Alexander Larkin. The first analysis stage combines an autonomous classifier with additional LLM-based analysis. The autonomous classifier runs locally on a CPU and is based on a backpropagation neural network. Together, these mechanisms produce a technical vulnerability description, identify likely weakness types, estimate CVSS severity, and provide input for ActionableScore. Two CVSS estimates are retained because incomplete kernel vulnerability information often permits more than one defensible interpretation: Conservative CVSS vector: AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H The Best / paranoid CVSS score: 5.5 The conservative vector represents a lower-impact interpretation. The Best/paranoid vector intentionally represents a plausible upper-bound interpretation and should not automatically be treated as demonstrated real-world impact. CVSS may also need to be adjusted for a particular Linux deployment, because actual reachability, privileges, enabled kernel configuration, hardware, namespaces, exposed device nodes, and other environmental conditions can differ significantly between systems. A separate ActionableScore mechanism evaluates practical remediation urgency. Its analysis may include reachability, attack prerequisites, subsystem exposure, memory-corruption characteristics, denial-of-service reliability, and possible confidentiality, integrity, or privilege-escalation impact. Conservative ActionableScore: unknown Paranoid ActionableScore: 0 The final base severity is taken directly from the second tab-separated field of the AL-KERNEL classification result. ActionableScore does not replace or independently override that final AL-KERNEL decision, and if ActionableScore adjusted impact level of ALKERNEL, then you would see self-readable flags above like INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7. For an AL-KERNEL result of MODERATE, this report uses the following additional presentation split: ActionableScore below 5 -> MODERATE REGULAR ActionableScore 5 or more -> MODERATE 7.0 The distinction between MODERATE REGULAR and MODERATE 7.0 makes it possible to identify Moderate issues that should receive manual analysis and fixes before lower-priority MODERATE REGULAR issues. In many cases, MODERATE REGULAR fixes may wait for a later rebase or routine update. There is one override in which MODERATE REGULAR becomes MODERATE 7.0 even when the ActionableScore is below 5. When the AL-KERNEL result contains the KPANIC flag, a MODERATE result is always presented as MODERATE 7.0. The KPANIC flag selected with few regexps without usage of AI at all, so it helps to detect cases when Kernel Crash happens and similar (to filter False-Negative results from the LLM usage). KPANIC indicates that a reliable kernel crash, kernel panic, or similarly serious kernel availability impact was identified by the classification workflow. AL-KERNEL base severity for this report: LOW KPANIC detected for this report: NO Published priority for this report (same as in Subject): LOW These results are intended to support engineering triage. They are machine-generated estimates, and cases marked for manual review should be validated by a human security engineer before final disposition. For more info read docs linked from here: https://kernelcve.org/ (and you can submit you own patch there to generate such a report for non-existant CVE-id yet). Note that in many cases this AI tool selects higher severity, than real is (means you can expect Importants instead of Moderate 7.0 or Moderates 7.0 instead of regular Moderates). If you see such cases, please use reply email interface to add additional manual analyses info to this particular CVE. And please, please, let me know when you see Lows instead of Importants or Important instead of Low (because particular for such cases I need to tune this AI tool to make it better for this one and next similar). My contact email for such notifications is alexanjelausa@gmail.com (and both send reply to CVE record itself too and see "reply" button below for howto reply).